Three-dimensional numerical simulations with
COBOLD, a new
radiation hydrodynamics code, result in a dynamic, thermally
bifurcated model of the non-magnetic chromosphere of the quiet Sun.
The 3D model includes the middle and low chromosphere, the
photosphere, and the top of the convection zone, where acoustic waves
are excited by convective motions. While the waves propagate upwards,
they steepen into shocks, dissipate, and deposit their mechanical
energy as heat in the chromosphere.
Our numerical simulations show for the first time a complex 3D structure of the chromospheric layers, formed by the interaction of
shock waves.
Horizontal temperature cross-sections of the model chromosphere
exhibit a network of hot filaments and enclosed cool regions.
The horizontal pattern evolves on short time-scales of the order of typically 20-25 s, and has spatial scales comparable to those of the
underlying granulation.
The resulting thermal bifurcation, i.e., the co-existence of cold and
hot regions, provides temperatures high enough to produce the observed
chromospheric UV emission and – at the same time – temperatures cold
enough to allow the formation of molecules (e.g., carbon monoxide).
Our 3D model corroborates the finding by [CITE]
that the chromospheric temperature rise of semi-empirical models does
not necessarily imply an increase in the average gas temperature but
can be explained by the presence of substantial spatial and temporal
temperature inhomogeneities.

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